Several authors write that ultra-high radio-frequency based identification (UHF RFID) of objects that are or contain electrically conductive material is challenging.
Mohammed et. al in their article “An RFID Tag Capable of Free-Space and On-Metal Operation” published by Information and Telecommunications Technology Center, University of Kansas, Lawrence, Kans. 66045, USA, propose an RFID tag that is approximately 142 mm×30 mm large and 3.18 mm thick. With regard to tags intended be used on a metal surface, the thickness is critical. An RFID tag having as its dimensions as proposed by Mohammed et al. could be used for laptop batteries if it were thinner and cheaper.
Eunni et al. in their article “A Novel Planar Microstrip Antenna Design for UHF RFID”, published in Systemics, Cybernetics and Informatics, Vol. 5, Number 1, propose a planar microstrip antenna design on which the antenna layer is placed on a substrate that is placed on a metallic object. The matching network has a vertical length of 12.4 mm and a horizontal length of 30 mm. Even though the size of the matching circuit alone is not of importance, it is the total size of the antenna is 140 mm×72 mm that matters. Normally, one tries to avoid a matching circuit in connection with an RFID tag, except when a broadband response is required.
Koivu in his thesis “Thin RFID Solution for Battery Identification”, EVTEK University of Applied Sciences, Institute of Technology, 2008, proposes a circular button tag configuration, in which an RFID tag is integrated in a polyethylene terephthalate (PET) sheet that is placed on a 150 μm thick paper sheet that is in turn placed on a ferrite sheet. As ferrite he uses magnetic sheets/radio wave absorbers of type Flexield IRLG4 manufactured by TDK, having thickness of 0.25 mm or 0.50 mm. The circular button tag is placed in corner of a mobile phone battery and he was able to achieve a read range of over four centimeters with all tested battery models if the tag was placed near the edges of the battery. A reading distance in this range can be considered to be sufficient for the intended use of remotely identifying mobile phone batteries.